1. Field of the Invention
The present invention is directed to a system for thermal energy injection-withdrawal, and more particularly a system for thermal energy injection-withdrawal for acquifers.
2. Description of the Prior Art
Aquifers are an underground porous media provided by nature through which water can flow. By heating or cooling water and transferring the water into an aquifer, a thermal storage or thermal energy sink is created. Access to the aquifer(s) is provided by a number of wells between the surface and the aquifer. For example, U.S. Pat. Nos. 3,931,851 and 3,965,972 suggest heating and cooling systems which use aquifer thermal energy storage. The literature has a number of heating and cooling systems using aquifer thermal energy storage prior to these patents.
Aquifers can provide extremely large thermal energy storage systems which have been produced by nature at a very economical cost to man. Most man-made large scale thermal energy storage systems cost one or more orders of magnitude more than the aquifer system. The cost of the thermal energy storage system is the series of walls between the underground aquifer and the surface. The thermal energy storage periods in an aquifer can be for a few hours up to a number of years. The percentage of energy recovered, even on an annual basis, is predicted to be higher than the performance of man-made devices.
A number of experimental tests and many limited applications of aquifer thermal energy storage have taken place. In some of these tests, the confining layers of the aquifer have ruptured upon water injection due to excess pressure. These pressures will be even greater for high temperature water which must be pressurized to prevent boiling. For example, 400.degree. F. water must be pressurized to 247 Psia to prevent boiling. If the water were allowed to boil, the temperature would drop very rapidly. If the pressure at the surface is 247 Psia, the pressure at the aquifer level is equal to 247 Psia plus the pressure due to the column of water in the injection pipe from the surface to the aquifer level. This pressure is much higher than the normal aquifer pressure, thereby requiring an extremely high injection flow rate to maintain pressure. If the pressure drops, the water boils and cavitation occurs. The aquifer and depth determine the minimum flow rate. This invention eliminates this restriction.
Similar excess pressure exists when the aquifer piezometric surface (water table) is far below the ground surface. The pressure due to the column of water from the surface to the piezometric surface is excess pressure. A high flow rate is again required to prevent cavitation.
In both the above cases, the injection water pressure is much higher than aquifer pressure. The pressure must be reduced to a pressure which provides injection flow rates rather than pressure differences caused by other related variables. If the injection pressures are excessive, the flow rate into the aquifer will be high and the water in the pipe to the surface will cavitate and/or the confining layers of the aquifer will fracture (blow out). Upon water withdrawal, the water pressure must be matched to required surface pressure.
It is not possible to store high temperature water in aquifers or to store cold water in deep aquifers at limited pressures without the invention herein described. Under these special conditions, water cannot be injected or recovered through an open tube injection/recovery system. Conventional pressure regulators will not provide required pressure reduction since a variable pressure as a function of flow rate is required.